Measurement of the composition of emulsions and the interface level of immiscible fluids is important in many applications. For example, it is important to characterize emulsions in oil field management. The measurement of the water and oil content of emulsions from individual oil wells may vary over the life of an oil field and may indicate the overall health of a field. In the case of injection wells, it is critical to control water quality to reduce hydrate formation and corrosion. Characterization of the composition of the oil and water mixture (e.g., measurement of the relative proportions of oil and water in the mixture) helps the operator improve well productivity and capacity. The information obtained is also useful to reduce back-pressure of wells, flowline size and complexity, and thermal insulation requirements.
Characterization of emulsions is also important in the operation of systems that contain fluids in a vessel (vessel systems) such as fluid processing systems. Vessel systems may include storage tanks, reactors, separators and desalters. Vessel systems are used in many industries and processes, such as the oil and gas, chemical, pharmaceutical, food processing industries, among others. For example, separation of water from raw oil is important to establishing production streams of oil and gas. Crude oil leaving the wellhead is both sour (contains hydrogen sulfide gas) and wet (contains water). The crude leaving the wellhead must be processed and treated to make it economically viable for storage, processing and export. One way of treating the raw oil is through the use of a separator. Most separators are driven by gravity and use the density differences between individual fluid phases of oil, water, gas, and solids to accomplish the separation. Identification of the interface levels of these layers is critical to the control of the separation process. Another fluid processing system where characterization of emulsions and measurement of the interface level is important is a desalter. Desalters are used in a refinery to control overhead corrosion downstream. In a desalter water and crude oil are mixed, inorganic salts are extracted into the water, and water is then separated and removed.
Finally, it is important to accurately characterize the water and salinity in the crude oil itself at various stages of the life of the product from a cost standpoint. Oil is a valuable commodity and underestimation of the water content in a typical tanker load can have significant cost consequences.
Wastewater management is another application where measurement and characterization of emulsion is important. Large quantities of oily wastewater are generated in the petroleum industry from both recovery and refining. A key factor in controlling the oil discharge concentrations in wastewater is improved instrumentation for monitoring the oil content of emulsions.
Many types of level and interface instruments have been contemplated over the years and a subset of those have been commercialized. Among those are gamma-ray sensors, guided wave sensors, magnetostrictive sensors, microwave sensors, ultrasonic sensors, single plate capacitance/admittance sensors, segmented capacitance sensors, inductive sensors, and computed tomography sensors. Each of the sensors has advantages and disadvantages. Some of the sensors are prohibitively expensive for many users. Some of the sensors may require a cooling jacket to perform at operating temperatures (above 125° C.). Some interface instruments require a clear interface to work, which can be problematic when working with diffuse emulsions. Some are susceptible to fouling. Other sensors do not have the ability to provide a profile of the tank, but rather monitor discreet points in the desalting process. Systems using electrodes are susceptible to the shorting of electrodes in high salinity applications and are susceptible to fouling. Finally, many of these systems are complex and difficult to implement.
Some existing sensor systems have used individual capacitive elements to measure fluid levels. A key limitation of those sensor systems is their inability to simultaneously quantify several components in the liquid. Capacitance methods have been used to measure dielectric constant of a liquid using specially designed electrodes for capacitance measurements. These designs are limited by the need for separate types of electrodes for capacitance measurements and for conductivity measurements. Inductor capacitor circuits also have been used to monitor the fluid level in a container using an electromagnetic resonator where change in capacitance was related to fluid level and fluid type. However, it has been the consensus of those of ordinary skill in the art that the filling of the resonator by a conducting liquid increased the uncertainties and noise in measurements by about one order of magnitude as compared to the values in a non-conducting fluid such as in air. However, these methods do not provide accurate measurements of concentrations of individual analytes at the limits of their minimum and maximum concentrations in the mixture.
With existing sensor systems, no one system is capable of delivering a combination of low cost, high sensitivity, favorable signal-to-noise ratio, high selectivity, high accuracy, and high data acquisition speeds. Additionally no existing system has been described as capable of accurately characterizing or quantifying fluid mixtures where one of the fluids is at a low concentration (i.e. at their minimum and maximum limits).